Xo. 493] 



EXPERIMENTS IN GRAFTING 



structure, if they form anything at all, namely, a leg, and 

 not a salamander to take the extreme case. But why 

 always the distal end of the leg and not the proximal, 

 i. e., not femur and scapula! The determination of the 

 distal end rather than the proximal must he due, I think, 

 to the presence of the free rounded knoli covered by the 

 new skin which gives the stimulus for a distal structure, 

 and the foot end of the leg is the only possible distal 

 structure that exists for this organ. 



The case is parallel to the formation of a heteromor- 

 phic tail in the earthworm, that develops, as I have 

 shown, from the anterior end of a piece when cut be- 

 hind the level of the twentieth segment, or thereabouts. 

 Here also a distal structure develops, but the nature of 

 the material is such that a tail rather than a head regen- 

 erates. While polarity, as an expression of the grada- 

 tion of the materials, is one of the factors that determines 

 a result, it is not the exclusive factor. In Lumbrieuhis 

 a head forms at the anterior end of a piece at nearly all 

 levels and a tail at the posterior end. Here we must 

 assume that the kinds of materials are so equally balanced 

 throughout the greater length of the worm that the po- 

 larity determines the result, while in the earthworm and 

 in the leg of the salamander another condition determines 

 a different result. In the latter cases the kind of mate- 

 rial, or the organ-complex, makes it possible for only a 

 tail or a leg to develop at either end of the piece, and 

 tli*- presence of a free end determines that its now strue- 



